Light weight flat heat pipe utilizing copper foil container laminated to heat treated aluminum sheets for structural stability

ABSTRACT

A very thin flat plate shaped sintered copper powder wick with a waffle pattern on one surface is sealed between two sheets of thin copper foil. The interior space within the sealed foil is evacuated and charged with sufficient water to saturate the wick through a copper tube which is then hermetically sealed, producing the working core of a flat plate heat pipe. Heat treated aluminum sheets are bonded with thin, thermally conductive adhesive layers to both the evaporator and condenser surfaces of the copper foil heat pipe container. The resulting flat heat pipe is lighter in weight by about 40%, much more durable, and less expensive to fabricate than all-copper, machined container flat plate heat pipes, while high performance is maintained.

BACKGROUND

[0001] 1. Field of Invention

[0002] Flat plate heat pipes or heat spreaders made of copper withsintered copper internal wicks and water as the working fluid arecurrently available. Copper-water is the best combination of containerand working fluid for heat pipes in the temperature range 20 to 100Cfrom a number of perspectives, including toxicity, flammability andperformance.

[0003] A significant drawback of this copper-water combination is theweight of the heat pipe resulting from the high density of copper (0.34lb/cubic inch), and its relatively low yield strength (about 10,000psi). A heat pipe made of copper approximately 8 in×12 in×0.25 in thickof necessity weighs about 3.5 lb.

[0004] I have invented a flat plate heat pipe with the benefits of thecopper-water combination, which is significantly lighter (about 2 lb.for the size described above), while also being stronger and moredurable and less expensive to produce.

[0005] My invention employs a copper foil internal container forcompatibility with water and a heat treated aluminum outer sheet surfaceto provide structural strength and durability.

[0006] 2. Description of Prior Art

[0007] Flat copper-water heat pipes are currently produced by severalmethods. One approach is to arrange multiple traditional cylindricalheat pipes in a parallel array soldered to a flat plate. A second layerof heat pipes may be arranged perpendicular to the first layer toachieve high heat flow in all directions, resulting in an isothermalcondensing surface.

[0008] Another method uses a machined copper container formed by a veryshallow pan about 0.2 inch deep with a grid of closely spaced supportsremaining after the pan has been machined from a 0.2 inch thick plate ofcopper. A copper powder wick is sintered into the pan and a fill tube issoldered or welded in place. A copper sheet covering the pan is thenwelded around the periphery of the resulting heat pipe container, whichis checked for leaks before being charged with an appropriate amount ofwater and sealed by clamping the fill tube and then welding it topermanently hermetically seal the finished flat heat pipe.

[0009] The drawbacks of the above-described approaches are the relativevulnerability to external insults resulting when one attempts to achievea light weight heat pipe. The large flat surfaces of the heat pipe, whenmade of copper, must be very thin to achieve a reasonably light weight.Internal supports (also solid copper) must be closely spaced to allowthe thin copper walls of the heat pipe to support even the externalatmospheric pressure (internal pressure of the heat pipe is very low).When this is done, the flat surfaces are quite fragile if bumped by asharp object. Machining of the copper container pan with support postsrequires significant time and cost.

[0010] U.S. Pat. No. 5,642,776 describes a very light weight heat pipewith a semi-rigid plastic foam wick. No protective outer surface sheetsare used, and internal vacuum would allow foil over vapor spaces in thewick to collapse, significantly impairing performance. It is therefore alow performance device, whereas the current invention approaches theultimate in flat heat pipe performance while reducing cost and weight.

[0011] U.S. Pat. No. 6,392,883 describes a flat heat pipe but gives nospecific guidance or performance information. The heat pipe discussed isa component of a multi-component heat dissipation system. No laminationsor wick details are taught.

SUMMARY

[0012] A flat plate copper-water heat pipe employs thin copper foil fora container to avoid the large weight penalty of a machined coppercontainer. A sintered copper powder wick with a waffle shaped gridmolded into one face provides mechanical support of the foil containerwhile the open space of the waffle grid allows free flow of steam tocool areas of the container surface for condensation heating. Thin heattreated aluminum sheets are bonded to both the evaporator and condensersurfaces of the copper foil container with a very thin film of thermallyconductive transfer tape, providing strength and durability whilepreserving high thermal performance.

OBJECTS AND ADVANTAGES

[0013] Accordingly, several objects and advantages of my invention areas follows. The weight of the copper-water heat pipe core is minimizedby utilizing thin copper foil to make the container, and by making thesintered copper wick (covering the entire evaporator surface of the flatheat pipe) as thin as is practical.

[0014] The space between the wick and the condensing copper foil surfaceis kept partially open to steam flow, both perpendicularly to the flatplate surface and laterally, by any of several means. Three such meansare 1) copper screen, 2) a grid molded into the sintered copper wick,and 3) a flat sheet of rigid copper open cell foam. Any of these serveto allow free flow of steam to any cool area of the heat pipe condensingsurface, keeping the condensing surface essentially isothermal even whenthe cooling load does not coincide with the heated area of theevaporator surface.

[0015] The heat pipe so constructed would not remain flat or bestructurally stable and durable without the addition of other elementsfor strength, stability and durability. This invention answers thisrequirement by laminating a much lighter and stronger sheet of heattreated aluminum to both faces of the flat heat pipe. The resultingcompleted flat heat pipe structure is reliably flat, much less fragileto damage by sharp objects, and most importantly weighs little more thanhalf the weight of a similar all-copper flat plate heat pipe.

[0016] Still further objects and advantages will become apparent from aconsideration of the ensuing description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an isometric view of my invention.

[0018]FIG. 2a is a partial cross-sectional view of one embodiment of myinvention indicated by section lines 2-2 of FIG. 1.

[0019]FIG. 2b is a partial cross-sectional view of another embodiment ofmy invention indicated by section lines 2-2 in FIG. 1.

[0020]FIG. 2c is a partial cross-sectional view of another embodiment ofmy invention indicated by section lines 2-2 of FIG. 1.

REFERENCE NUMERALS

[0021]10 flat plate heat pipe invention

[0022]12 condensing surface aluminum sheet

[0023]14 evaporating surface aluminum sheet

[0024]16 potting material

[0025]18 sintered copper powder waffle surface wick

[0026]20 copper foil sheet

[0027]22 copper foil pan

[0028]24 transfer tape

[0029]26 copper screen

[0030]28 flat sintered wick

[0031]30 rigid copper open cell foam sheet

[0032]32 hermetic seal

Preferred Embodiment—Description

[0033]FIG. 1 shows the present flat plate heat pipe invention 10 withcondensing surface aluminum sheet 12. Evaporating surface aluminum sheet14 is on the underside of FIG. 1 and only two of its edges are shown inthis view. Potting material 16 comprised of epoxy or other elastomericmaterial provides a smooth edge around the periphery of the laminatedflat plate heat pipe invention 10.

[0034]FIG. 2a depicts a partial cross-sectional view of flat plate heatpipe invention 10 in the direction of the section arrows of FIG. 1.Sintered copper powder waffle surface wick 18 is made by sinteringwithout compaction in a hydrogen atmosphere at 850C for about one halfhour. Copper powder has particle size of about 0.05 to 0.1 mm diameterbefore sintering. The waffle surface of sintered copper powder wafflesurface wick 18 is formed by sintering in a machined graphite orstainless steel mold. The wick is about 0.04 to 0.20 inch thickover-all, with the waffle grid stand-offs about 0.03 to 0.15 inch thick.The waffle grid standoffs are about 0.06 to 0.25 inch round or squarewith the grooves between them about 0.04 to 0.25 inch wide. The openspace formed by the grooves is about 50% to 80% of the area of the solidportion of the wick. Sintered copper powder waffle surface wick 18 maybe sintered to copper foil sheet 20 or it may be simply held in place byexternal atmospheric pressure (internal working pressure of the heatpipe is only about 1 psi absolute pressure). The copper foil heat pipecontainer is made by welding copper foil pan 22 to copper foil sheet 20around their periphery, forming a hermetic seal 32. Copper foil ofcopper foil sheet 20 and copper foil pan 22 is approximately 3 to 5ounce per square foot (0.004 to 0.007 inch thick). A copper evacuationand charging tube (not shown), about 0.06 to 0.12 inch diameter, iswelded, soldered or brazed in place through the side wall of copper foilpan 22 for leak checking and charging with a small amount of water, theworking fluid.

[0035] The copper foil heat pipe container is bonded to evaporatingsurface aluminum sheet 14 and condensing surface aluminum sheet 12 withvery thin (about 0.002 inch thick) transfer tape 24 with or withoutceramic filler for improved thermal conductivity, such as 3M VHB orthermal transfer tape. Aluminum sheets 12 and 14 are about 0.03 to 0.12inch thick. The bonding process may be accomplished under vacuum toachieve full surface area bonding. Other means for mechanically andthermally joining the copper foil container to the outer heat treatedaluminum sheets, such as ultrasonic welding, may alternatively be used.

[0036] Final leak checking with a helium mass spectrometer leak checkerand charging with an appropriate amount of pure, degassed and deioniizedwater may be done either before or after bonding the aluminum sheets tothe copper container. The water charge volume is about equal to the openinterstitial spaces of the sintered copper wick. After charging withwater, the fill tube is sealed by clamping and welding to produce apermanent hermetic seal.

[0037] Epoxy cement or other potting material 16 is then applied to fillthe voids and provide a smooth edge around the periphery of flat plateheat pipe invention 10.

Preferred Embodiment—Operation

[0038] In operation the object of the invention is to transfer heat fromthe warmer evaporating surface aluminum sheet 14 to the coolercondensing surface aluminum sheet 12 while maintaining the entire areaof condensing surface aluminum sheet 12 at a controlled uniformtemperature. Heat is applied to evaporating surface aluminum sheet 14 bymeans such as an electrical etched foil resistance heater. Ideally theheated area should correspond to the area of condensing surface aluminumsheet 12 to be warmed, but a principal benefit of flat heat pipes is theability to efficiently spread heat laterally while maintainingessentially a uniform temperature over the entire surface of condensingsurface aluminum sheet 12.

[0039] Sintered copper powder waffle surface wick 18 is saturated withpure water which fills the microscopic voids in the wick, holding thewater in the wick by capillary attraction regardless of heat pipeorientation. The open region of the waffle surface is filled with watervapor only, and for temperatures below about 90C the vapor pressure ofwater is much less than one atmosphere.

[0040] When heat is added to evaporating surface aluminum sheet 14, ittransfers by conduction through evaporating surface aluminum sheet 14,transfer tape 24, and copper foil sheet 20 to sintered copper powderwaffle surface wick 18. Heat added to water in the wick causes somewater to be vaporized to steam which leaves the wick and flows to anycooler region of condensing surface of copper foil pan 22 where itcondenses and heats the surface by releasing its latent heat ofvaporization. Heat is then transferred by conduction through copper foilpan 22, transfer tape 24, and condensing surface aluminum sheet 12 toits cool surface where heat is needed. Condensed liquid water isabsorbed into the wick and flows by capillary action to refill the voidsleft by water that has evaporated into steam.

[0041] Steam will only condense on surfaces cooler than the steam, sothat heat is only applied where it is needed. A temperature measuringsensor in condensing surface aluminum sheet 12 acts through atemperature controller to turn heat to evaporating surface aluminumsheet 14 on and off as needed.

Other Embodiments

[0042] Copper Screen Spacer—Description

[0043]FIG. 2b shows an alternative means for providing a relatively openregion for steam to flow to cool areas of copper foil pan 22 where itcondenses and warms the cool area. In this embodiment a copper screen 26separates flat sintered wick 28 from copper foil pan 22. Copper screen26 serves the function of waffle grid stand-offs of FIG. 2a. In otherdetails, this embodiment is similar to the preferred embodiment.

[0044] Copper Screen Spacer—Operation

[0045] In operation this embodiment is similar to the preferredembodiment, except that the flow of steam from wick to condensingsurface is through the open spaces between the wires of copper screen26.

[0046] Rigid Copper Foam Spacer—Description

[0047]FIG. 2c shows another alternative means of providing a relativelyopen region for steam flow. In this embodiment a rigid copper open cellfoam sheet 30 separates flat sintered wick 28 from copper foil pan 22.Rigid copper open cell foam sheet 30 serves the function of waffle gridstand-offs of FIG. 2a. In other details, this embodiment is similar tothe preferred embodiment.

[0048] Rigid Copper Foam Spacer—Operation

[0049] In operation this embodiment is similar to the preferredembodiment, except that steam flows from the wick through the open cellsof rigid copper open cell foam sheet 30.

Conclusions, Ramifications, and Scope

[0050] Accordingly, it can be seen that the laminated flat plate heatpipe of this invention provides marked improvements over existing art byeliminating much machining of the heat pipe container, thereby reducingmanufacturing cost. Heat treated aluminum sheet outer layers greatlyincrease resistance to denting and puncture, thus producing a much moredurable product. High thermal performance is preserved by maintainingvery low thermal resistance through the laminations. The transfer tapeadhesive is very thin, and its thermal conductivity may be enhanced byceramic additives. Another very important benefit of this invention isthe reduction in weight of about 40% compared to an all-copper flat heatpipe.

[0051] Although the description above contains many specificities, theseshould not be construed as limiting the scope of the invention but asmerely providing illustrations of some of the presently preferredembodiments of this invention. Various other embodiments andramifications are possible within its scope. Thus the scope of theinvention should be determined by the appended claims and their legalequivalents, rather than by the examples given.

What is claimed is:
 1. A flat plate heat pipe comprising: A.) a flatplate-shaped wick for absorbing and holding water by capillaryattraction, B.) a copper foil hermetically sealable container extendingslightly beyond the edges of and enclosing said wick, C.) means forholding said wick slightly spaced apart from one face of said copperfoil container while allowing free flow of steam throughout regionbetween said spaced apart wick and said face of copper foil container,D.) means for evacuating, leak checking, and charging said containerwith water, such as a copper tube passing through the wall of saidcontainer and hermetically soldered, brazed, welded or otherwise sealedthrough said wall, E.) two thin flat sheets of highly heat conductive,mechanically strong material slightly larger in extent than saidcontainer and placed above and below said foil container and closelycontacting the surfaces of said container, F.) means for thermallyconductively and mechanically attaching said flat sheets to said copperfoil container, whereby a light weight, mechanically durable, low costflat copper-water heat pipe may be produced.
 2. The flat plate heat pipeof claim 1 wherein said wick is made of copper powder sintered togetherto form a mechanically rigid structure.
 3. The flat plate heat pipe ofclaim 2 wherein said sintered wick has on one of its flat plate surfacesa waffle-shaped pattern molded into said sintered wick, the projectionsof said waffle-shaped pattern serving as said means for holding saidwick spaced apart from said face of copper foil container.
 4. The flatplate heat pipe of claim 3 wherein said projections of said waffleshaped pattern are about 0.06 to 0.25 inch round or square and about0.03 to 0.15 inch in height with open grooves between said projectionsbeing about 0.04 to 0.25 inch wide, whereby about 50% to 80% of wicksurface area remains open for free flow of steam.
 5. The flat plate heatpipe of claim 2 wherein said means for holding said wick spaced apart ismade of copper screen.
 6. The flat plate heat pipe of claim 2 whereinsaid means for holding said wick spaced apart is made of rigid open cellcopper foam.
 7. The flat plate heat pipe of claim 1 wherein said twothin flat sheets of highly heat conductive, mechanically strong materialare made of heat treated aluminum.
 8. The flat plate heat pipe of claim1 wherein said means for thermally and mechanically attaching said flatsheets to said copper foil container is very thin transfer tapeadhesive, either plain or ceramic-filled to increase thermalconductivity.
 9. The flat heat pipe of claim 1 wherein said copper foilcontainer is made of copper foil about 0.004 to 0.008 inch thick. 10.The flat plate heat pipe of claim 1 wherein said copper foil containeris made of one flat copper foil sheet and one pan-shaped copper foilsheet with pan sides being equal in height to the combined thickness ofsaid wick and said means for holding wick spaced apart from said face ofcopper foil container, A.) said pan-shaped copper foil sheet having anarrow flange around its entire periphery and parallel to large flatarea of said pan-shaped copper foil sheet, the edges of said flangecorresponding in extent to the edges of said flat copper foil sheet, B.)said edges of said sheets being hermetically joined together by welding,soldering, brazing or other means, whereby the heat pipe container maybe easily assembled.